109 lines
4.5 KiB
Go
109 lines
4.5 KiB
Go
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package basic
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import (
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"math"
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"sync"
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"sync/atomic"
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)
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// 视恒星时是 UT 的纯函数，但月掩全球路径会在同一条计算链里反复向它求值：
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// 地球自转、升落上下文、测地投影各自按自己的调用点重算同一个瞬时。实测单场 Saturn
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// 2025-01-05 的请求里，191,517 次求值只对应 51,308 个不同的儒略日（重复距离中位数只有
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// 2 次调用），而每次求值都要完整算一遍 77 项 IAU2000B 章动。
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//
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// 这里用一张有界直接映射表把结果记下来：无分配、容量固定（4096 槽 × 24 字节），
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// 用 RWMutex 保证 C 共享库被宿主多线程调用时安全。表项记录写入时的 ΔT 世代，
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// 因此 astro.SetDeltaT 覆盖之后旧条目自然失效，不会返回陈旧恒星时。
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//
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// Apparent sidereal time is a pure function of UT, yet one occultation path query evaluates
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// it many times for the same instant from independent code paths (Earth rotation, rise/set
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// contexts, geodetic projection). A single Saturn 2025-01-05 request performed 191,517
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// evaluations for only 51,308 distinct Julian days, and each evaluation ran the full
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// 77-term IAU2000B nutation. This bounded direct-mapped memo removes that redundancy without
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// allocating: a fixed 4096-slot table guarded by an RWMutex, with the ΔT generation stored in
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// each entry so an astro.SetDeltaT override invalidates stale values instead of replaying them.
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// 4096 槽对单场月掩的 5 万余个不同儒略日而言明显偏小（重复距离中位数只有 2 次调用），
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// 这里扩到 16384 槽（16384×24 B = 384 KB，BSS 静态数组，不参与初始化）。
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const siderealMemoBits = 14
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const siderealMemoSize = 1 << siderealMemoBits
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type siderealMemoEntry struct {
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key uint64
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value float64
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generation uint64
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}
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var (
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siderealMemoMu sync.RWMutex
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siderealMemoTable [siderealMemoSize]siderealMemoEntry
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siderealMemoHits uint64
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siderealMemoMisses uint64
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siderealMemoStores uint64
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)
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// siderealMemoIndex 用高低位混合避免相邻儒略日落在相邻槽位而互相驱逐。
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// siderealMemoIndex mixes high and low bits so adjacent Julian days do not evict each other.
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func siderealMemoIndex(jd float64) uint64 {
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bits := math.Float64bits(jd)
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return (bits ^ (bits >> 29)) & (siderealMemoSize - 1)
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}
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// siderealMemoLoad 返回缓存命中值；ΔT 世代不匹配时按未命中处理。
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// siderealMemoLoad returns a cached value; a generation mismatch counts as a miss.
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func siderealMemoLoad(jd float64) (float64, bool) {
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generation := deltaTGenerationValue()
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entry := &siderealMemoTable[siderealMemoIndex(jd)]
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siderealMemoMu.RLock()
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key, value, entryGeneration := entry.key, entry.value, entry.generation
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siderealMemoMu.RUnlock()
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if entryGeneration == generation && key == math.Float64bits(jd) {
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atomic.AddUint64(&siderealMemoHits, 1)
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return value, true
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}
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atomic.AddUint64(&siderealMemoMisses, 1)
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return 0, false
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}
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// siderealMemoStore 只在 ΔT 世代未变时写入：世代必须在**求值前**采样（见 siderealMemoGeneration），
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// 否则求值期间发生的 SetDeltaTFn 会把旧 ΔT 的结果打上新世代并长期回放。
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// siderealMemoStore writes only while the ΔT generation is unchanged; the generation must be sampled
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// before the evaluation, otherwise a SetDeltaTFn during the computation would stamp the old value
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// with the new generation and replay it.
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func siderealMemoStore(jd, value float64, generation uint64) {
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if deltaTGenerationValue() != generation {
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return
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}
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index := siderealMemoIndex(jd)
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siderealMemoMu.Lock()
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siderealMemoTable[index] = siderealMemoEntry{
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key: math.Float64bits(jd),
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value: value,
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generation: generation,
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}
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siderealMemoMu.Unlock()
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atomic.AddUint64(&siderealMemoStores, 1)
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}
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// siderealMemoGeneration 在求值前采样 ΔT 世代，供 siderealMemoStore 校验。
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func siderealMemoGeneration() uint64 {
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return deltaTGenerationValue()
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}
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// siderealMemoStats 返回命中/未命中/写入计数，供测试守护记忆表确实生效。
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func siderealMemoStats() (hits, misses, stores uint64) {
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return atomic.LoadUint64(&siderealMemoHits), atomic.LoadUint64(&siderealMemoMisses),
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atomic.LoadUint64(&siderealMemoStores)
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}
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func resetSiderealMemo() {
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siderealMemoMu.Lock()
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for i := range siderealMemoTable {
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siderealMemoTable[i] = siderealMemoEntry{}
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}
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siderealMemoMu.Unlock()
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atomic.StoreUint64(&siderealMemoHits, 0)
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atomic.StoreUint64(&siderealMemoMisses, 0)
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atomic.StoreUint64(&siderealMemoStores, 0)
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}
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